HEAT PUMPS SIMULTANEOUS HEATING AND COOLING
District system AM and PM loads – 4 May
200 180 160 140 120 100 80 60 40 20 0
Simultaneous load less than 50% of system capacity for short periods
charge a thermal store and release this to augment or provide the afternoon cooling and vice versa. The drawback of this system, however, is
Cooling kWh Heating kWh
Figure 5: Morning and afternoon loads with small periods of simultaneous loads
heat pumps. The same then occurs during the cooling cycle in the afternoon, where the following morning’s heating can be generated during the afternoon cooling process (see fi gure 7). Effectively, load shifting generates as
much energy as possible simultaneously, thereby hugely increasing overall system effi ciency. Use the morning heat load to
that the hydraulics of the heat pump may need to be separated from the LTHW and CHW systems as, by virtue of the three port valves, the fl uids from the ground array will pass to the LTHW and CHW. If there is a thermal transfer fl uid requiring antifreeze to protect the evaporator, this fl uid will be present in both the LTHW and CHW systems. This is an added installation and maintenance cost. A further drawback is the control strategy required to operate such a system. Generally, the heat pumps can only deliver one load to a controlled set point. For example, if the system is cooling dominant at a certain point in time, the heat pumps are programmed to deliver the load to a set point in cooling. The heating then provided by the condenser is effectively ‘uncontrolled’, in that the heat pump is not delivering this load to a particular set point. The building management system (BMS) must therefore identify how best to deliver this load and when, using the three port
Simple COP analysis Full heating or full cooling only Pumps and compressors 28 kW
Load 100 kW System COP 3.57
100 kW
Full simultaneous heating and cooling
Pumps and compressors 28 kW Load 200 kW System COP 7.14
Three port valves used to direct free cooling or free heating to the plate exchangers
Simultaneous capabilities using three port valves
100 kW 50 kW 0.75 kW
Heat pump evaporator ‘cold side’
50 kW 0.75 kW
12.5 kW input
HP 1
12.5 kW input
HP 1
50 kW 0.75 kW
Heat pump condenser ‘hot side’
50 kW 0.75 kW
Heat pumps control to either heating or cooling load and the opposite load is provided free by diverting three port valves. They do not ‘reverse’, they simply have a hot and a cold side
Ground array Figure 6: A simultaneous heating and cooling heat pump system with three port valves
56
CIBSE Journal November 2012
www.cibsejournal.com
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